The present invention is directed generally towards gas turbine combustors and more specifically towards an apparatus and method of providing a self-purging pilot fuel injection system.
A typical gas turbine engine comprises at least a compressor, a turbine, and at least one combustor. The compressor takes air from the surrounding atmosphere and compresses it by directing it through a plurality of stages of rotating and stationary airfoils, raising its pressure and temperature. This compressed air is then directed into a combustion system, which is most commonly annular or can-annular, and comprises a plurality of fuel injectors. Fuel, either gas or liquid, is mixed with the compressed air and ignited to form hot combustion gases. The hot combustion gases are then directed through a multi-stage turbine, which is coupled to the compressor, and for power generation, to an electrical generator.
The combustion system of the gas turbine engine typically has a number of operating points, depending on the power output required from the engine. The various operating points can generate different emissions levels, especially carbon monoxide (CO) and oxides of nitrogen (NOx). As a result, the combustion system will have different fuel injection points in order to maintain emissions levels within acceptable standards given the different power output requirements. Therefore, depending on the power output requirement, a higher emissions level operating point may be required.
A majority of combustion systems operate in one or both of two modes: diffusion and premix. Premix combustion systems offer lower emissions levels due to their ability to premix the fuel and air prior to igniting the mixture. On the contrary diffusion combustion systems operate where fuel and air mix along the flame front to produce a diffusion flame. That is, there is essentially no mixing prior to combustion. As a result, molecules of fuel remain unburned and result in higher level of emissions. However, some combustion systems utilize both modes of operation in that they employ a diffusion type mode during engine start-up, since a diffusion combustion system has a richer fuel content and results in greater starting reliability. Once these engines have started they transfer to a premix operation for extended periods of running so as to produce lower emissions.
An area of concern with any type of combustion system is the issue of flashback. Flashback occurs when pressures within the combustion system fluctuate such that a flame can travel upstream from the combustion zone into a premixing zone or fuel injection region. This is especially of concern for pilot fuel nozzles that historically have high fuel-air concentrations and that are fuel rich to support a pilot flame. Fuel injection adjacent to and in direct contact with the flame zone typically requires purging to ensure that when the fuel is shut-off the flame does not travel up a fuel line. Should there be a leak in a fuel line, hot gas can be drawn back up the fuel line and ignite, causing extreme damage and possible failure of the combustion hardware.
The present invention seeks to overcome the shortfalls of the prior art by providing an apparatus and method of self-purging a pilot fuel injection system that also provides an ignition source for a combustor.